Archive for November, 2012

After opening my 350S and giving it a good clean, I decided to carry out a few simple mods before putting it back together again.

The first thing I did was to detach the external connections to the two circuit boards to make it easier to take everything apart and get at. These connections were:

keyboard

speaker

power

styluses

pitch

I carefully desoldered the wires, and replaced them with 2 or 3-way Molex connectors, like these:

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At this stage I decided not to make any further modifications to the keyboard. The keyboard PCB now plugs into the main circuit board and is much easier to remove for cleaning and for further potential modifications.

This picture shows the front and back of the keyboard PCB with a 2-way Molex socket fitted, connecting top and bottom of the chain of resistors which produce the different notes:

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In the case of the speaker wires, the Molex connector makes it easier to move the main circuit board around while working on it, as the wires are no longer than absolutely necessary and the speaker is firmly fixed to the top half of the Stylophone body.

The modification I made to this – which I’ve done with several of my instruments recently – was to add a switch to swap between the internal speaker and a larger external speaker (as described here). I chose a large DPDT rocker switch, which seemed to be in keeping with the 350S’s style. I don’t know how necessary it was, but as the internal speaker is 35Ω and the external speaker is 8Ω , I added a 3W 27Ω resistor in series with the output, which is a pair of 4mm banana plug sockets.

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As far as power was concerned, I first wanted to replace the large PP9 batteries. Not only are these heavy and expensive, but they take up a lot of room inside the Stylophone case, which might be needed to house extra circuitry. So what I decided to do at this stage was to replace them with something more practical: rechargeable PP3’s.

I wasn’t sure these would be powerful enough to allow the 350S to function properly, but I exchanged the PP9 wiring for PP3-sized battery clips and everything seemed to be working. I then looked for some PP3 holders that would provide a more permanent fixture for the batteries. This type seemed to fit the bill:

There was just enough room to fit these side by side into one of the covers formerly used for access to the PP9’s, each one attached with 4 small nuts and bolts. Although opened from the outside, these battery holders occupy the internal space originally taken up by one of the PP9’s.

Clips for the two PP3’s are connected to the power Molex connector via a 3.5mm mono socket with an integral switch, so that anything plugged into the socket automatically disconnects the internal batteries.

Later, the socket might be used for an 18v power supply, but for the time being I attached the discarded PP9 wiring and clips to a 3.5mm plug, so that PP9’s can still be used, but don’t have to be installed inside the body of the 350S.

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Unlike the regular Stylophone, the 350S has two styluses: one for normal playing, sounding continuously for as long as the stylus is in contact with the keyboard; and one for ‘Reiteration’ mode – with the appropriate switch selected – producing a fast or slow series of pulses, in imitation of a banjo or mandolin, on which it’s common to pluck a single note repeatedly.

However, I had found while modifying normal stylophones, that it was sometimes handy to have two styluses, one in each hand, for playing quicker or more intricate passages; so I decided to rewire the two existing styluses as standard, and add two extra ones for Reiteration mode.

With the Molex connectors in place, it was easy to wire all four styluses up, but not so easy to find a way to secure the extra two to the Stylophone in such a way that they would be easy to reach. In the end, I used a pair of clips like this, sold on eBay as penholders and meant, I think, to clip onto a pocket:

I had some spare white styluses, so the ‘normal’ styluses are black, and the ‘reiteration’ styluses are white. I attached a holder each side of the Stylophone in which the white styluses sit. The long wires attached to these can be pushed inside the body of the Stylophone when not in use.

I wasn’t able to find an exact match for the wire used by Dübreq for attaching the styluses. It’s only just over 2mm in diameter, and very flexible; there are no more than 10 or 11 strands of wire inside quite a thick outer layer, and a non-conductive cord running along the length of it, on the inside – presumably for strengthening. If I ever find out where to get it, I’ll add it as an Edit to this post: in the meantime I had to make do with a standard multi-stranded white ‘hook-up’ wire of about the same width.

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Dealing with the pitch of the 350S didn’t involve detaching external wires, in fact, but I added a 3-way Molex connector to the tuning control to make it easier to experiment with.

Unlike some of my previous Stylophone mods, I wasn’t looking for extreme pitch changes this time, but something more along the lines of a synth modulation wheel. Strangely, these seem to be very rare, but I found one produced by the German company Doepfer, described here. It comes as a kit of parts, like this:

The pot supplied with it is a 10k, which has a knurled shaft fitting tightly inside the hole in the ‘half-wheel’.

I wired the wheel in parallel with the existing tuning control, and its effectiveness depended on three things:

1. The setting of the tuning control: the higher it was set, the less variation produced by the wheel. Not much I could do about this, as the tuning control is used to set the 350S to the correct pitch, compared to other instruments. If it proves a problem, it could perhaps be solved in the future by a slight adjustment to the keyboard resistor chain.

2. The value of the pot. I found that a 2.5k pot was the most effective, but couldn’t find one with a shaft compatible with the Doepfer wheel. So I added some 10k resistors in parallel with the 10k pot. Originally I added 3, which would have made the pot 2.5k, but 2 seemed to be enough (3.3k), and took up less space, so I left it at that.

3. The part of the potentiometer track covered by movement of the wheel. The wheel wasn’t able to move the wiper of the potentiometer round the whole track – which is normal for mod wheels, joysticks, etc. It took a bit of experimentation to find the right place, which essentially meant turning the potentiometer to exactly the right position before attaching the wheel. It needed to be at zero when the wheel was deflected fully down, and eventually I found the right place, wired leads and a Molex plug to it and fixed it in place with small nuts and bolts.

The whole construction took the place previously occupied by the left-hand PP9, with the wheel appearing through a slot in the top of the 350S . The Doepfer kit cost about £10, so it was a bit of an extravagance, and something like it could probably be rigged up more cheaply. However, it adds an interesting feature to the 350S which it never had before.

This picture shows the pitch wheel assembly in place and also, in the background, the speaker switch and banana sockets.

This picture shows the top half of the 350S body, with the new components and the Molex connectors in place, with the circuit boards removed:

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After fitting everything, it was time to put the 350S back together.

The first item to go back into place was the main PCB. This picture shows the board in position, with the 6 fixing screws marked:

Next, the Keyboard PCB was installed. The 4 fixing screws are marked:

Before the bottom half of the 350S body was attached, the PP3 battery clips were fed into the battery holders:

The two halves of the body were fitted together and batteries inserted:

Finished and ready to go! The underside of the 350S now looks like this:

and the front and back like this:

This gives a good view of the power socket on the back left, as you look at it; the speaker switch and sockets on the back right; the pitch wheel on the top on the right; and the white ‘reiteration’ styluses in their holders.

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Finally, with the 350S back together and in operation, I looked at the suggested external addition, a volume pedal. According to the 350S manual, this would replace the photo control, and adjust not only the volume, but also the waa filter and the vibrato depth.

The manual recommends a ‘standard Foot Pedal’: but what was a standard foot pedal in the 1970s is not what we might consider a standard foot pedal – or ‘expression’ pedal – these days. What’s required here is a 50k-100k log pot, which plugs into the 350S via a 6.35mm (1/4″) mono jack plug.

I had an old volume pedal (probably dating almost from that era!) which I was able to adapt. The original cable was crackly and the pot was scratchy, so I shortened the cable to remove the section that was obviously damaged inside, and replaced the pot.

I didn’t have a 47k or 50k to experiment with, so I used a 100k, but that seemed to be fine. The only oddity is that the ‘waa’ works backwards, in that the filter is at the ‘high end’ with the heel down – as compared to, for example, a guitar wah pedal, where heel-down is the low end, and toe-down is the high end. I tried putting a polarity change switch in the pedal, but that didn’t work, as the pedal mechanism – just the same as the pitch wheel described earlier – is set to reach its minimum when the heel is fully down, and doesn’t cover the full travel of the pot, so when the two ends of the pot were swapped, the pedal wasn’t reaching zero, which it needed to do to produce the full ‘waa’ effect. I’ll just have to get used to it.

After playing the instrument for a while, I noticed that one of the switches was a bit crackly, so this is something I might tackle later on, together with a couple more mods I have in mind.